1. Field of the Invention
This invention relates to improvements in the conversion of aromatics. More particularly, it relates to the regeneration of aromatics processing catalysts. This invention especially relates to the regeneration of zeolite catalysts employed in aromatics conversions.
2. Background of the Invention
The prior art is replete with processes relating to the manufacture of aromatic compounds having six to eight carbon atoms, namely benzene, toluene and xylene (BTX). At the present time, the most valuable of these is p-xylene, which may be separated for use in synthesis of polyesters from mixed xylenes by fractional crystallization or by selective adsorption. Also highly valued is benzene for use as chemical raw material. Toluene is also valuable for varied uses as a solvent in chemical manufacture and as a high octane gasoline component.
Typically, p-xylene is derived from mixtures of C.sub.8 aromatics separated from such raw materials as petroleum naphthas, particularly reformates, usually by selective solvent extraction. The C.sub.8 aromatics in such mixtures and their properties are:
______________________________________ Freezing Boiling Density Lbs./ Point .degree.F. Point .degree.F. U.S. Gal. ______________________________________ Ethylbenzene -139.0 277.0 7.26 P-xylene 55.9 281.0 7.21 M-xylene -54.2 282.4 7.23 O-xylene -13.3 292.0 7.37 ______________________________________
Principal sources are catalytically reformed naphthas and pyrolysis distillates. The C.sub.8 aromatic fractions from these sources vary quite widely in composition but will usually be in the range 10 to 32 wt. % ethylbenzene with the balance, xylenes, being divided approximately 50 wt. % meta, and 25 wt. % each of para and ortho.
Individual isomer products may be separated from the naturally occurring mixtures by appropriate physical methods. Ethylbenzene may be separated by fractional distillation although this is a costly operation. Ortho-xylene may be separated by fractional distillation and is so produced commercially. Para-xylene is separated from the mixed isomers by fractional crystallization or by selective adsorption.
As commercial use of para and ortho-xylene has increased there has been interest in isomerizing the other C.sub.8 aromatics toward an equilibrium mix and thus increasing yields of the desired xylenes. At present, several xylene isomerization processes are available and in commercial use.
The isomerization process operates in conjunction with the product xylene or xylenes separation process. A virgin C.sub.8 aromatics mixture is fed to such a processing combination in which the residual isomers emerging from the product separation steps are then charged to the isomerizer unit and the effluent isomerizate C.sub.8 aromatics are recycled to the product separation steps. The composition of isomerizer feed is then a function of the virgin C.sub.8 aromatic feed, the product separation unit performance, and the isomerizer performance.
It will be apparent that separation techniques for recovery of one or more xylene isomers will not have material effect on the ethylbenzene introduced with charge to the recovery/isomerization "loop". That compound, normally present in eight carbon atom aromatic fractions, will accumulate in the loop unless excluded from the charge or converted by some reaction in the loop to products which are separable from xylenes by means tolerable in the loop. Ethylbenzene can be separated from the xylenes of boiling point near that of ethylbenzene by extremely expensive "superfractionation". This capital and operating expense cannot be tolerated in the loop where the high recycle rate would require an extremely large distillation unit for the purpose. It is a usual adjunct of low pressure, low temperature isomerization as a charge preparation facility in which ethylbenezene is separated from the virgin C.sub.8 aromatic fraction before introduction to the loop.
Other isomerization processes operate at higher pressure and temperature, usually under hydrogen pressure in the presence of catalysts which convert ethylbenzene to products readily separated by relatively simple distillation in the loop, which distillation is needed in any event to separate by-products of xylene isomerization from the recycle stream. For example, the Octafining catalyst of platinum on a silica-alumina composite exhibits the dual functions of hydrogenation/dehydrogenation and isomerization.
In Octafining, ethylbenzene reacts through ethyl cyclohexane to dimethyl cyclohexanes which in turn equilibrate 5 to xylenes. Competing reactions are disproportionation of ethylbenzene to benzene and diethylbenzene, hydrocracking of ethylbenzene to ethane and benzene and hydrocracking of the alkyl cyclohexanes.
The rate of ethylbenzene approach to equilibrium concentration in a C.sub.8 aromatic mixture is related to effective contact time. Hydrogen partial pressure has a very significant effect on ethylbenzene approach to equilibrium. Temperature change within the range of Octafining conditions (830.degree. to 900.degree. F.) has but a very small effect on ethylbenzene approach to equilibrium.
Concurrent loss of ethylbenzene to other molecular weight products relates to percent approach to equilibrium. Products formed from ethylbenzene include C.sub.6 + naphthenes, benzene from cracking, benzene and C.sub.10 aromatics from disproportionation, and total loss to other than C.sub.8 molecular weight. C.sub.5 and lighter hydrocarbon by-products are also formed.
The three xylenes isomerize much more selectively than the reaction of ethylbenzene, but they do exhibit different rates of isomerization and hence, with different feed composition situations the rates of approach to equilibrium vary considerably.
Loss of xylenes to other molecular weight products varies with contact time. By-products include naphthenes, toluene, C.sub.9 aromatics and C.sub.5 and lighter hydro-cracking products.
Ethylbenzene has been found responsible for a relatively rapid decline in catalyst activity and this effect is proportional to its concentration in a C.sub.8 aromatic feed mixture. It has been possible then to relate catalyst stability (or loss in activity) to feed composition (ethylbenzene content and hydrogen recycle ratio) so that for any C.sub.8 aromatic feed, desired xylene products can be made with a selected suitably long catalyst use cycle.
A different approach to conversion of ethylbenzene is described in U.S. Pat. No. 3,856,872 of Morrison. Over an active acid catalyst comprising a crystalline zeolite characterized by a silica to alumina mole ratio of at least about 12 and a constraint index within the approximate range of 1 to 12, typically described as a ZSM-5 type zeolite, ethylbenzene disproportionates to benzene and diethylbenzene which are readily separated from xylenes by the distillation equipment needed in the loop to remove by-products. It is recognized that the rate of disproportionation of ethylbenzene is related to the rate of conversion of xylenes to other compounds, e.g. by disproportionation. See also U.S. Pat. No. 3,856,873 of Burress which also describes reaction of C.sub.8 aromatics over ZSM-5 and shows effects of various temperatures up to 950.degree. F. in the absence of metal co-catalyst and in the absence of hydrogen.
In the known processes for accepting ethylbenzene to the loop, conversion of that compound is constrained by the need to hold conversion of xylenes to other compounds to an acceptable level. Thus, although the Morrison technique provides significant advantages over Octafining in this respect, operating conditions are still selected to balance the advantages of ethylbenzene conversion against the disadvantages of xylene loss by disproportionation and the like.
A further advance in the art is described in U.S. Pat. No. 4,163,028 of Tabak, et al., which discloses xylene isomerization and ethylbenzene conversion at high temperature with ZSM-5 of very high silica/alumina ratio whereby the acid activity is reduced.
A more recent development of Tabak, et al., is found in U.S. Pat. No. 4,236,996 which discloses a low acidity zeolite catalyst, typified by ZSM-5, which has been steamed at high temperature to reduce its activity. In using this less active catalyst the temperature is raised to above 700.degree. F. to attain xylene isomerization, preferably to 800.degree. F. or higher. At these temperatures, ethylbenzene reacts primarily via dealkylation to benzene and ethane (or ethylene in the absence of hydrogen and hydrogenation co-catalyst) rather than via disproportionation to benzene and diethylbenzene and hence is strongly decoupled from the catalyst acid function. Since ethylbenzene conversion is less dependent on the acid function, a lower acidity catalyst can be used to perform the relatively easy xylene isomerization, and the amount of xylenes disproportionated is eliminated. The reduction of xylene losses is important because about 75% of the xylene stream is recycled in the loop resulting in an ultimate xylene loss of 6-10 Wt. % by previous processes. Since most of the ethylbenzene goes to benzene instead of benzene plus diethyl benzenes, the product quality of the new process is better than that of prior practices.
In addition to xylene isomerization, other aromatic conversion processes have gained importance. U.S. Pat. Nos. 3,126,422; 3,413,374; 3,598,878; 3,598,879 and 3,607,961 show vapor-phase disproportionation of toluene over various catalysts.
The disproportionation of aromatic hydrocarbons in the presence of zeolite catalysts has been described by Grandio et al. in the Oil and Gas Journal, Vol. 69, No. 48(1971).
The use of a catalyst comprising a crystalline zeolite characterized by a silica to alumina mole ratio of at least about 12 and a constraint index within the approximate range of 1 to 12 for the disproportionation of toluene is described in many patents, such as U.S. Pat. Nos. 4,011,276, 4,016,219, 4,052,476, 4,097,543, and 4,098,837. Other aromatic conversions such as transalkylation, cracking, alkylation and hydrocracking are typically conducted with this type of catalyst.
These catalytically promoted processes have a disadvantage found in many catalytic processes, catalyst activity declines due to deposition of "coke", a carbonaceous material, on the catalyst which progressively masks the active sites of the porous zeolite catalyst. The coke can usually be removed by burning with a molecular oxygen containing gas to regenerate the activity of the catalyst.
The regeneration of solid contact material of catalytic and non-catalytic nature contaminated with combustible deposits is taught in the prior art. U.S. Pat. No. 2,391,327 of Mekler discloses the regeneration of catalysts contaminated with heavy combustible materials with a cyclic flow. A regenerating gas stream passes through the contaminated catalyst, then through heat recovery and purifying equipment, through equipment where the free oxygen content, the temperature and other variables are adjusted to desired values and then back to the catalyst bed for further reactivation of the catalyst.
U.S. Pat. No. 3,755,961 of Francis et al. relates to the regeneration of coke-containing crystalline zeolite molecular sieves which have been employed in an absorptive hydrocarbon separation process. The process involves the continuous circulation of an inert gas containing a quantity of oxygen in a closed loop arrangement through the bed of molecular sieves. To prevent damage to the molecular sieve bed from water vapor, one of the combustion products of the regeneration, it is removed from the inert gas stream before the inert gas is recycled to the inlet to the molecular sieve bed. Commonly, the water vapor is removed by passage of the gas stream through a bed of water-lean water adsorbent. In addition, the circulatory gas stream is cooled by indirect heat exchange in an air or water cooled heat exchanger. Gas is vented from and inert gas and air are added to the circulating gas stream, as required.
Where catalyst activity declines rapidly, continuity of operation is achieved by the well-known "swing reactor" technique. In this procedure, two or more reactors are employed, one of which is on stream, while burning regeneration is conducted on a reactor containing spent catalyst which has lost activity by coke deposition. Cycles of two to four days or even less are common practice in this technique using one reactor on stream for that period and then shifting to a freshly regenerated vessel.
Longer operating times between regeneration are commercially desirable and have been attained in aromatic conversions. Cycles of several weeks or several months are not uncommon and operating runs as long as a year or more have been obtained. The use of specially prepared catalysts of controlled activity, the incorporation of metal into the zeolite catalyst and the addition of hydrogen to the reaction mixture have been some of the techniques employed in the prior art to obtain prolonged cycle times.
It has been taught heretofore that steam and high temperature like those encountered during regeneration to burn off coke were detrimental to zeolite structure and catalytic activity. The prior art aromatic processes taught the use of recycle gas driers to minimize exposure of the zeolite containing catalyst to water vapor formed during regeneration.
In direct contradiction to these prior art teachings, recycle gas driers are expressly not utilized in the regeneration process disclosed in commonly assigned patent application Ser. No. 121,340, filed Feb. 14, 1980. In this application, a catalyst comprising zeolite, which has a silica to alumina mole ratio of at least about 12 and a constraint index within the approximate range of 1 to 12, is regenerated in the presence of steam at water partial pressures of between about 0.1 psi and about 4.0 psi, at a contact time of between about 12 and about 72 hours and a temperature of between about 750.degree. and about 900.degree. F. The catalyst regenerated in this fashion can have an activity enhanced many times greater than its original activity which can be translated to longer cycle times. The regeneration process of said application controls the regeneration conditions to mildly steam the catalysts so as to enhance their activity, rather than to deactivate them. Oxidizing gas is passed to the reactor containing the bed of coked catalyst where it reacts with the coke to form a hot combustion gas stream. This hot gas stream is removed from the reactor, passed to a condenser and then to a separator operated at a temperature of about 35.degree.-150.degree. F. where liquid effluent is removed. The cooled gaseous stream containing water vapor is then introduced to a heater together with the required quantity of make-up oxidizing gas and the heated gas stream is recycled to the reactor for further regeneration of the catalyst.
It is an object of this invention to provide an improved process for regenerating an aromatics conversion catalyst comprising a zeolite.
It is another object of this invention to regenerate an aromatics conversion catalyst without employing a gas cooler and a gas-liquid separator.
It is a further object of the invention to regenerate a ZSM-5 type zeolite xylene isomerization catalyst of controlled low acid activity.